In lighting applications, the luminaires are required to provide a certain angular light distribution. Additionally, it is often desired that the luminaire exit window appears uniformly lit. That is, the luminous emittance of the exit window should be spatially uniform. This means that the luminous emittance Mv({right arrow over (ρ)}, θ0, φ0) at the angles θ0 and φ0 of the exit window should not depend on the position {right arrow over (ρ)} within that exit window. The luminous emittance at the angles θ0 and φ0 is obtained by integrating the luminance of the exit window Lv({right arrow over (ρ)}, θ0, φ0) over a solid angle. The solid angle corresponds to a human eye pupil size at some typical distance from the exit window. Because the angular pupil size ΔΩ(θ0) is small, the luminous emittance as a function of the coordinate {right arrow over (ρ)} within the exit window, observed from the inclination angle (polar angle) θ0 and the azimuth angle φ0, can be expressed as follows:Mv({right arrow over (ρ)},θ0,φ0)=Lv({right arrow over (ρ)},θ0,φ0)ΔΩ(θ0)OK,na  (1)
The total luminous emittance (integrated over all angles) Mv({right arrow over (ρ)}) or simply the luminous emittance is given byMv({right arrow over (ρ)})=∫Lv({right arrow over (ρ)},θ,φ)dΩ  (2)
The requirement to provide a specific angular light distribution is the main optical problem of lighting. Combined with the uniformity requirement, it becomes a significant challenge. This challenge is made more difficult by the trend to replace other light sources with Light-Emitting Diodes (LEDs). Although LED luminaires can provide a significant reduction in energy consumption, their exit windows can appear visually unusual and less appealing than traditional luminaires. The reason is the geometrical form-factor of the LED light sources: they are relatively small, creating highly non-uniform luminous emittance (typically observed as a “spotty” or “dotted” pattern) of the luminaire's exit window. This may even cause unpleasant glare effects.
It is desirable to provide luminaires that are less spotty. To date, many of the attempts to achieve this have involved reflecting light from the exit window back into the luminaire. This reflection is associated with optical losses and lower optical efficiency, which conflicts with the desire to save energy.